1. Field of the Invention
The present invention relates to a driving system for a vibrating type actuator for driving a member to be driven by vibrating a vibrating body, and a method of driving the vibrating type actuator.
2. Related Background Art
As a vibrating type actuator for driving a member to be driven to move linearly, various actuators have been proposed. For example, Japanese Patent Application Laid-Open No. 2004-320846 discloses a linear ultrasonic actuator.
The above-mentioned linear ultrasonic actuator will be described with reference to FIGS. 6, 7, 8, 9 and 10. FIG. 6 is a perspective view schematically showing a configuration of a conventional linear ultrasonic actuator. FIG. 7 is a plan view showing a pattern of electrodes formed on a piezoelectric element shown in FIG. 6 as an electrical/mechanical energy transformation device. FIGS. 8A and 8B are perspective views schematically showing a first mode and a second mode, respectively, which are vibrating modes of a vibrating body shown in FIG. 6. FIG. 9 is a view schematically showing an oval motion occurring in projecting portions of the vibrating body shown in FIG. 6. FIG. 10 is a block diagram showing a driving circuit for driving the linear ultrasonic actuator shown in FIG. 6.
A linear ultrasonic actuator 17 includes a vibrating body 10 as shown in FIG. 6. The vibrating body 10 includes an elastic body 101 in a plate shape which is made of a metallic material, and a piezoelectric element 102 fixed to one surface of the elastic body 101. On the other surface of the elastic body 101, a pair of projecting portions 101a rising vertically are formed, and the respective projecting portions 101a are arranged in a longitudinal direction of the elastic body 101 at a predetermined interval therebetween. A moving body 12 made of a magnet is pressed by its magnetic force to be in contact with the respective projecting portions 101a. A method of pressing the respective projecting portions 101a and the moving body 12 is not limited to a method due to a magnetic force, and may be a method of pressing with an elastic member such as a spring.
On an exposed surface (surface opposed to the surface fixed to the elastic body 101) of the piezoelectric element 102, two electrodes Va and Vb arranged in the longitudinal direction are formed, as shown in FIG. 7. On the surface of the piezoelectric element 102 to which the elastic body 101 is fixed, a common electrode Vc is formed over an entire surface of the piezoelectric element 102, and the common electrode Vc is connected to a ground (common electric potential). The piezoelectric element 102 is previously polarized so that the polarities of a region overlapping the electrodes Va and Vb become the same in the thickness direction, or the polarities of the region overlapping the electrodes Va and Vb become opposite in the thickness direction. Herein, an example will be described in which the polarities of the region overlapping the electrodes Va and Vb are the same in the thickness direction.
When alternate current signals in phase with the same frequency are applied respectively between the electrode Va and the common electrode Vc and between the electrode Vb and the common electrode Vc, of the piezoelectric element 102, a vibration in the second mode as shown in FIG. 8B occurs in the vibrating body 10. The vibration in the second mode is a primary bending vibration (flexural vibration) in a short-side direction of the vibrating body 10. Furthermore, when alternate current signals in a reversed phase are applied respectively between the electrode Va and the common electrode Vc and between the electrode Vb and the common electrode Vc, a vibration in the first mode as show in FIG. 8A occurs in the vibrating body 10. The vibration in the first mode is a secondary bending vibration (flexural vibration) in a long-side direction of the vibrating body 10. The vibration occurring in the first mode is different from the vibration occurring in the second mode not only in the direction in which an antinode and a node are arranged, but also in the position of a node. To be specific, the direction in which an antinode and a node are arranged in the first vibration mode is orthogonal to that in the second vibration mode.
Herein, the vibrating body 10 is configured so that resonance frequencies at a time of the vibration in the first mode and the vibration in the second mode are substantially matched with each other. When alternate current signals, which have frequencies close to the above-mentioned resonance frequency, and whose phases are neither in phase and nor reversed phase, are applied to the respective electrodes Va and Vb, the vibrations in the first mode and the second mode, each of which has a phase difference of π/2(rad) or −π/2(rad), occur in the vibrating body 10. At this time, each projecting portion 101a of the vibrating body 10 is formed at a position where the node portion of the vibration in the first mode is matched with the antinode portion of the vibration in the second mode, so that an oval motion as shown in FIG. 9 occurs on each projecting portion 101a of the vibrating body 10. Owing to the oval motion, the moving body 12 moves linearly in its longitudinal direction.
The driving circuit for driving the linear ultrasonic actuator 17 includes an alternate current signal generator 13, a phase shifter 14, and two step-up circuits 15 and 16, as shown in FIG. 10. The alternate current signal generator 13 generates two alternate current signals in phase with the same frequency. The phase shifter 14 outputs one alternate current signal (alternate current signal corresponding to the second mode) output from the alternate current signal generator 13 while shifting the alternate current signal by a predetermined phase amount. Herein, a shift amount φ of a phase is in a range satisfying a relationship of 0<φ<π. The step-up circuit 16 steps up the voltage of the alternate current signal output from the phase shifter 14 to a voltage at which the linear ultrasonic actuator 17 is operable. The step-up circuit 15 steps up the voltage of the other alternate current signal (alternate current signal corresponding to the first mode) output from the alternate current signal generator 13 to a voltage at which the linear ultrasonic actuator 17 is operable. The alternate current signal with the voltage stepped up by the step-up circuit 15 is applied to the electrode Va of the piezoelectric element 102, and the alternate current signal with the voltage stepped up by the step-up circuit 16 is applied to the electrode Vb of the piezoelectric element 102. Furthermore, the above-mentioned driving circuit is connected to a ground in the same way as in the common electrode Vc of the piezoelectric element 102.
In the driving circuit thus configured, the shift amount of a phase in the phase shifter 14 is adjusted, thereby making it possible to change an amplitude ratio of two vibration modes of the vibrating body 10, i.e., an oval ratio of the oval motion shown in FIG. 9. As a result, the drive speed and the drive direction of the linear ultrasonic actuator 17 with respect to the moving body 12 can be controlled.
However, in order to drive the above-mentioned conventional linear ultrasonic actuator, the above-mentioned circuit for generating alternate current signals in two phases and circuits for stepping up the voltages of the respective alternate current signals are required. This enlarges the circuit scale of the driving circuit, making it difficult to configure the driving circuit at a low cost.